The present invention relates, in one aspect, to a thermal transfer sheet for printing letters, figures, patterns, etc. having metallic feeling such as metallic luster to a transfer-receiving material, and more particularly, to a thermal transfer sheet capable of providing metallic feeling without being influenced with high heat generated at a time of printing by a thermal printer. In another aspect, the present invention also relates to a thermal transfer sheet capable of providing metallic feeling without being influenced with a surface condition of a transfer-receiving material such as paper even in a case where images or the like are preliminarily printed on the transfer-receiving material and the printed surface has irregularity because of the presence of an printed ink layer or a case where the transfer-receiving material has flat smooth surface portion and non-flat smooth surface portion in combination.
In a prior art, there is known a hot stamping method, as a method of printing letters or figures having metallic luster, in which a transfer foil is heat pressed on the surface of the transfer-receiving material by using a stamp formed of such as metallic material provided with a protrusion having the same pattern as a design of a printed matter.
There is utilized, as such a transfer foil, a layered product formed by laminating a peeling (or peelable) layer, a deposition anchor layer, a metal deposition layer and an adhesive layer in this order on one surface of a substrate film having a thickness of about 9 to 25 .mu.m. The adhesive layer side of the transfer foil, that is, most outer surface of the transfer foil, faces the surface of the transfer-receiving material such as paper, and the stamp heated to a temperature of about 100 to 130.degree. C. is pressed against the transfer-receiving material from the substrate film side of the transfer foil for several seconds to thereby transfer a desired image on the surface of the transfer-receiving material.
However, according to such hot stamping method, it is necessary to produce the metal stamp in accordance with every image to be printed, and therefore, much cost is involved even in a case of less printed materials required, providing an economical problem. Furthermore, in order to carry out a halftone recording, it is necessary to make dots and the like on the surface of the stamp with high precision, and there is required much time and labour and also caused a problem that it is difficult to record fine halftone images.
In recent years, there has been provided a thermal transfer recording method using a thermal head and a thermal transfer ribbon. This thermal transfer recording method uses a heating element such as thermal head in place of a stamp utilized in the hot stamping method mentioned above, and this method is suitable for obtaining a small amount of the printed matters. Furthermore, according to the thermal transfer recording method, the halftone image can be easily recorded through a so-called area gradation method in which concentration gradation is expressed by controlling the area ratio of a dyed or colored portion with respect to a printed area, for example, by changing the sizes of dots to be applied respective portions. For this reason, it is desired to print letters or figures having metallic luster through such thermal transfer recording method.
For example, Japanese Patent Laid-open (KOKAI) Publication No. SHO 63-30288 or No. HEI 1-257082 discloses a technology such that a thermal transfer process is performed by the thermal head with the use of an metallic foil obtained through improvement of that used for the conventional hot stamping method. In these publications, there is disclosed a transfer foil improved by applying a thin substrate film to the conventional transfer foil for hot stamping method which is formed by laminating the peeling layer, the deposition anchor layer, the metal deposition layer and the adhesive layer in this order on one surface of the substrate film, or also disclosed a transfer foil improved by making the deposition anchor layer to serve a function as peeling layer.
However, in such conventional transfer foil for the hot stamping method, the deposition anchor layer is formed of a resin material such as acrylic group resin, urethane resin, cellulose resin or the like, and the above described transfer foil improved for the thermal transfer process using the thermal head also has the similar deposition anchor layer. For this reason, when such conventional transfer foil is used for the transfer process using the thermal head, the metal deposition layer loses its metallic luster, i.e. is clouded, at the time of printing and it is impossible to obtain a recorded material having a mirror-like metallic luster appearance.
Such loss of the metallic luster of the metal deposition layer is caused by a difference in processes at a time of thermal energy application between the thermal transfer process using the thermal head and the hot stamping process. That is, in the hot stamping process, a recorded material is obtained by pressing the stamp heated to a temperature of about 100 to 130.degree. C. for several seconds from the back surface side of the substrate film having a thickness of about 9 to 25 .mu.m. On the other hand, in the thermal transfer process using the thermal head, a recorded material is obtained by pressing the thermal head from the back surface side of the substrate film having a thickness of about 3 to 6 .mu.m, and then the temperature of the thermal head surface is increased to about 300.degree. C. in several to ten-several m.sec. Accordingly, in the process of using the thermal head, the deposition anchor layer formed on the substrate film is heated to a temperature at least about 130 to 200.degree. C. even in consideration of thermal energy loss. At this time, when the deposition anchor layer is formed of a conventional resin material as mentioned above, the deposition anchor layer is heated to a temperature more than a glass transition temperature, and because of pressure further applied, elastic deformation or plastic deformation will be caused. In such case, the metal deposition layer formed on the deposition anchor layer as a mirror-like surface cannot follow up the deformed deposition anchor layer, generating a number of fine cracks in the metal deposition layer. As this result, in a printed material formed through the thermal transfer process to the transfer-receiving material, a number of fine cracks will be caused on the metal deposition layer and the surface thereof will be clouded.
The transfer foils mentioned above include one prepared by using a two liquid setting (curing) type or one liquid setting type resin as a material for the deposition anchor layer, applying such resin on the substrate film surface and then carrying our the thermosetting process to thereby increase the glass transition temperature of the deposition anchor layer. In the case of using the two liquid or one liquid setting type resin, although there causes no problem of the loss of the metallic luster of the metal deposition layer, it is not suitable for the application to the coating on the thin substrate film usable for the thermal transfer process using the thermal head because of short pot life at the coating time or setting condition of high temperature and long time, thus providing a problem.
Furthermore, for the conventional transfer foil for the hot stamping process, an adhesive layer is formed of a mixture of a wax group material and an adhesive resin or formed of a resin capable of increasing a cohesive strength of the adhesive layer, and even for the transfer foil improved for the thermal transfer process using the thermal head, an adhesive layer similar to that mentioned above is used.
Incidentally, in a case where an image having luster is formed on a paper having no flat smooth surface, i.e. irregular surface, it is necessary for the conventional adhesive layer of the structure mentioned above to include a wax component having low melt viscosity so as to infiltrate into recessed portions of the transfer-receiving material surface. However, when an adhesive layer having much wax component is used, the adhesive layer itself loses the cohesive strength, so that the metal deposition layer is easily peeled and removed because of cohesive failure of the adhesive layer after the transferring to the transfer-receiving surface. In such case, it may be possible to improve the adhesive property between the metal deposition layer and the transfer-receiving material by making thin the thickness of the adhesive layer, but it becomes impossible to absorb the irregularity of the transfer-receiving material by the adhesive layer, and accordingly, level difference may appear on the transfer-receiving material surface or cracks may be caused thereon, losing the luster. Therefore, if it is attempted to form an image having good luster on the paper having no flat smooth surface, an excellent luster appearance cannot be expected.
On the other hand, in a case where an image having luster is formed on a transfer-receiving material such as a film which has relatively flat smooth surface and into which the adhesive layer material less infiltrates, an adhesive effect due to the infiltrating force of the adhesive layer material is hardly expected. For this reason, in order to well maintain the fixing property of the printed matter, it is necessary to increase the resin component in the adhesive layer and to increase the cohesive strength of an ink. However, in the case of increased resin component, strength of the adhesive layer is excessively increased to lower the printing sensitivity and the resolution. This problem may be somewhat improved by making high the sensitivity of the resin component, i.e. making low the molecular weight, or making low the glass transition temperature (Tg). However, in such treatment, sheet blocking may be easily caused at a time when the thermal transfer sheet is fed in roll form.
Furthermore, in a case when a transfer-receiving material having a highly flat smooth surface is used, the printing sensitivity, the resolution and the fixing property of the metal deposition layer can be extremely improved by making thin the thickness of the adhesive layer. However, in a case when a transfer-receiving material on which another print has already been formed, the surface of the transfer-receiving material provides irregularity even if the transfer-receiving material has itself a flat smooth surface and, hence, an adverse effect is given to the luster of the metal deposition layer as like as in the case of the transfer-receiving material having no flat smooth surface. Particularly, in recent years, since commercial packing papers and commercial labels are formed with many designs, there are many cases where images having metallic luster are further formed in an overlapped manner to coat papers, plastic films, synthetic papers or the like on which printed images have already been formed. Furthermore, the fixing property of the printed matter can be improved by the strength of the adhesive layer containing increased resin component. However, also in such case, the material forming the adhesive layer does not infiltrate in the portions having difference in level at a boundary portion between the printed portion and the non-printed portion, so that such portion provides further worse adhesive property.